It appears I'm in denial. There I was, smug at how much running I do, gloating that Government diktats on healthy living don't apply to me. But apparently I'm no better than a heroin addict. Scientists reported last week that "excessive running shares similarities with drug-taking behaviour".
One expects this kind of thing about extreme sports, whose devotees relish the association of being called "adrenalin junkies". But the researchers were talking about running, the same activity encouraged by virtually everyone in the health industry.
They found that too much of it sparks a reaction in the brain that is similar to heroin – and it is just as addictive. It gets worse. Sudden withdrawal can lead to trembling, writhing and teeth chattering.
Writing in the medical journal Behavioural Neuroscience, the researchers found that a desire to get off the sofa and shed a few pounds can quickly become as compulsive as Class A narcotics. So mild exercise like jogging can develop into a serious triathlon or marathon habit. "Although exercise is good for your health, extreme exercise may be physically addictive," they warned.
Tell me about it. My story began with a simple desire to get fit. I signed up to the London Marathon. Like many people, I found the mix of camaraderie, excitement and finish-line euphoria a heady cocktail – and I was hooked.
Once the blisters had healed and the memory of painful muscles had subsided, I had one abiding thought. What next? Before I knew it I was on a one-way slippery slope to harder stuff – ultra marathons, five-day, non-stop races and extreme triathlons.
In what has become a familiar rite of passage for thousands of Britons, I signed up to the Marathon des Sables, a 150-mile run across the Sahara desert. This is where City boys, the SAS and the likes of Ben Fogle come to test themselves.
A behavioural psychologist would have had a field day among the 600 entrants; many have addictive personalities. In my tent was a former alcoholic ex-para who'd swopped booze for extreme physical tests. He told us proudly how he'd once done 10,000 sit-ups – in a day.
Rory Coleman is typical of many reformed characters who've done the race. Fifteen years ago, he was an overweight alcoholic with a 40-a-day smoking habit. Told by a doctor that he'd be lucky to make 40, he traded the pub for a pair of trainers.
Three months later, he ran a half marathon, then a year after that the London Marathon. Since then, the 47-year-old has clocked 619 marathons. A normal week sees him running 100 miles. "I'm somebody that needs exercise," he says. "I don't ever intend to stop. But I'm not addicted to running," he claims. "I've just made it a part of my life. And it's a positive thing – have you ever met a heroin addict who says they enjoy it?"
Mimi Anderson, 47, says her friends often comment that she's just swopped one addiction for another. For fifteen years she suffered from anorexia. Now an "ultra-runner", she regularly competes in marathons in excess of 100 miles. Last year she broke the female John O'Groats to Land's End running record, covering the 840 miles in 12 days and 15 hours (averaging 65 miles a day).
Like the rats that were denied exercise in the study, she says she gets twitchy after a week of no training. "Normally I run seven days a week, but my husband has told me I can only do six. He gets really cross with me. I've just got back from one race and I'm about to do another. It is an addiction," the grandmother concedes. "But it's a healthy one."
It's also highly intoxicating. In the endurance athlete bible, Survival of the Fittest, Dr Mike Stroud explains how opiate drugs like heroin create artificially what the body produces naturally. In other words, if you want to get high, forget heroin, take up running.
The cocktail of drugs the body produces include the pain-relievers endorphins and dopamine (also produced during orgasm), the anti-depressant serotonin and the "fight or flight" hormone adrenalin, which increases strength and concentration. It's quite a cocktail.
When the finish line of the Marathon des Sables came into view after running 150 miles, it was as though someone rammed a needle of adrenalin in my chest. One minute I was hobbled over like an early hominid, stumbling on bruised, battered and blistered feet. The next I was like Usain Bolt, sprinting to the finish line, arms aloft.
Extreme running can also induce the same effects of amphetamines. Last year, 48 hours into a non-stop (no-sleep) race across Ireland, I started hallucinating. A team-mate had to drag me away from jabbing a rock with my walking pole. I was convinced it was a deposit box full of money.
This brings a novel argument in the debate against drugs. No one has stood up and pointed out an uncomfortable truth, that they're a poor substitute. A cocaine high apparently lasts a mere 15 minutes. Big deal. After a race, I can be high for days, surfing on a wave of euphoria.
But as the researchers found, withdrawal leads to a comedown: depression, apathy, listlessness. Like the rats in the study, I'm suffering from it at the moment. It's been a while since my last big event, a 100km run around Mt Blanc, and I'm starting to crawl up the wall.
There's a 280km run across the Alps next month that takes my fancy. Or maybe the Ben Nevis Triathlon, a 1.9km swim, 90km bike ride, and a quick run up and down Ben Nevis. That should sort me out. Yes, there may be an addictive element to all this. But recently I was told by a doctor that I have the lungs of someone ten years younger. As addictions go, I can think of worse.